Bulk spontaneous polarization, dynamical charges(Z*)and piezoelectric stress tensor elements of tetragonalPbZr1/2Ti1/2O3(PZT 50/50) with B-site cations ordered along [001] and [111] directions andPbTiO3(PT) were determined from ground-state Berry’s phase calculations using the all-electron Linearized Augmented Plane Wave (LAPW) method. Extensive internal strain contributions to the piezoelectric response in PT are due to large transverse effective charges and sensitivity of atomic positions to lattice strains. Theoretical proper moduli of PT,e15=3.15C/m2,e31=−0.93C/m2ande33=3.23C/m2,agree well with single crystal experimental data. Ordering along the polar [001] direction was found to enhance thee33piezoelectric stress modulus in chemically ordered PZT 50/50. Theoretical proper moduli in this latter material differ significantly from low temperature experimental data, indicating the possibility that the mechanism of piezoelectric response in PZT near the morphotropic phase boundary differs from conventional single crystal piezoelectricity found in PT. ©1998 American Institute of Physics.